Method and apparatus for recovering carboxylic acids from dilute solutions

Abstract: A method for recovering carboxylic acids from a dilute aqueous solution thereof having a concentration below about ten percent (10%) by weight, which includes passing the dilute acid solution through a reverse osmosis separator, thereby producing a permeate substantially free of acid and a retentate having an acid concentration above about ten percent (10%) by weight. The retentate is contacted with a liquid extractant for acids to produce an acid-rich extractate and an acid-free raffinate. The acid is then recovered from the acid-rich extractate.

Description: This invention relates to methods and apparatus for recovering carboxylic acids from dilute solutions thereof by the use of reverse osmosis membrane systems in combination with other acid recovery techniques. Many industrially important processes result in the production of streams of carboxylic acids in dilute concentrations. Economic considerations and environmental concerns lead to the desirability of recovering carboxylic acids from such streams for reuse, for sale, or for disposal in the most economic manner. Experience has shown that carboxylic acids in dilute concentrations, i.e. below about fifteen percent (15%) by weight, in aqueous streams, are most economically removed and recovered through the use of a liquid-liquid extraction process employing a high-boiling solvent. When the carboxylic acids make up less than about three percent (3%) by weight of the stream to be treated, it becomes difficult to recover the acids economically by such a liquid-liquid extraction process, although environmental considerations often make it necessary to treat such streams nonetheless. US Patent No 2 953 502 describes the use of a non-porous membrane to produce mixtures having differing portions of the components from the starting mixture. The mixtures can then be distilled. US Patent No 3 182 043 describes a membrane which is resistant to acids and so can be used to separate water from aqueous solutions of monocarboxylic acids. Patent Abstracts of Japan, Vol 7. No. 225(C-189) 137060 dated 1983 describes a reverse osmosis membrane for use in concentrating an organic acid. It does not, however, teach any after-treatment of the resulting concentrated acid. In accordance with the present invention, there is provided a method of recovering carboxylic acids from a dilute solution thereof, comprising passing the dilute solution through a reverse osmosis membrane separator to produce an aqueous permeate substantially free of acid and aqueous retentate having an acid concentration more concentrated than the initial concentration of the dilute acid solution, characterised in that the carboxylic acids have from 1 to 7 carbon atoms or are mixtures thereof, the dilute acid solution has an acid concentration below about 10% by weight, and the aqueous retentate has an acid concentration above about 10% weight, and in that the retentate is treated in an acid separation step by contacting the retentate with a liquid extractant for the acids thereby to produce an acid rich extractate and a substantially acid free raffinate, and recovering acid from the acid rich extractate . Thus dilute carboxylic acid streams having a low concentration of acids are concentrated following which the relatively more concentrated streams so produced are further treated, as by liquid-liquid extraction to recover the carboxylic acids they contain with better economic performance. The retentate from the acid separation step is subjected to a further separation process, preferably liquid-liquid extraction, to produce an extractate, which in turn can be subjected to further acid separation steps, preferably distillation, to recover a purified and concentrated carboxylic acid product stream. It is preferred that the extractant employed in the liquid-liquid extraction step be high-boiling material, for example secondary and tertiary amines such as those sold under the trademarks Adogen by Sherex, Amberlite by Rohm & Haas, Hosterex by Hoechst, or alkyl phosphine oxides sold under the trademark Cyanex by Cytec Industries. Other separation processes besides liquid-liquid extraction which may be used following the membrane separation process include extractive distillation, azeotropic distillation and classical distillation. The method of the invention is particularly effective when the carboxylic acids are formic, acetic, propionic, and butyric acids and mixtures thereof and most preferably carboxylic acids having from two to four carbon atoms. In a preferred form of the invention, the carboxylic acid is recovered from the acid-rich extractate by a distillation step, which may be a classical distillation or an extractive distillation. In further preferred form of the invention, the dilute aqueous solution of carboxylic acids is subjected to a preliminary treatment step prior to being subjected to reverse osmosis separation in order to remove impurities such as aromatic carboxylic acids, metallic ions, free metal particles, organic or inorganic salts, straight chain carboxylic acids having more than five (5) carbon atoms, and sugars, along with other similar impurities. The impurity removal step may be performed by filtration, microfiltration, ultrafiltration, nanofiltration, or a combination of two or more of such processes. The membrane separator employed may utilize a semi-permeable membrane formed from a material such as polysulphone, polyether sulphone, cellulose acetate, polyamide, cellulose esters, cellulose triacetate, polyether urea, polyvinyl alcohol, polypiperiazamide, polyfuren, cellulose acetate butyrate, aromatic polyamide, polyethyleneimine, and thin film composites of any of the above. The single figure of the drawing illustrates in block diagram form a flow diagram for a preferred process of the invention. In the figure, the process streams flow generally from left to right through the system. The equipment for practising the invention includes a pre-treatment section 20, which may be a single or multiple stage filtration, microfiltration, ultrafiltration, or nanofiltration device, or a combination of such devices. If the stream to be treated is sufficiently clean, the pre-treatment step or stage 20 may be omitted. Preferably, another pre-treatment stage 22, which is preferably a nanofiltration stage, is employed in the process of the invention. One of the output streams from the nanofiltration step, the filter concentrate, comprises a concentrated stream of heavy impurities, which are withdrawn from the system in known manner and directed to recover equipment or are otherwise disposed of. Such withdrawal is shown as occurring through line 24. The next step in the preferred operation of the invention is a membrane separator stage 26. The equipment includes a semi-permeable membrane having the property of rejection carboxylic acids in large part and passing water. The output streams from the reverse osmosis stage 26 include a retentate stream 28 and a permeate stream 30. The permeate stream 30 is substantially pure water and is passed to disposal or reuse within the plant in which the process is being practiced. If desired, the permeate may be passed through additional reverse osmosis stages similar to stage 26 or forming a part thereof to enhance the recovery of carboxylic acids which may be residually present in the permeate stream. Similarly, the retentate passing through retentate stream 28 may be subjected to additional reverse osmosis treatment to produce a higher concentration of carboxylic acids in the retentate stream, and to produce subsidiary permeate streams which may be combined with permeate stream 30, or disposed of separately or returned separately for use within the plant in which the process of the invention is being practiced. The retentate passing through line 28 is then subjected to a liquid-liquid extraction treatment in extraction-recovery-purification stage 32, and the raffinate resulting from such treatment leaves the system through line 34. The raffinate is substantially pure water and may be disposed of or reused in the plant in which the invention is being practiced. The acid-rich extractant may be separated by distillation or by other recovery and purification methods which may involve, if desired, multiple stages. Such treatments eventually produce purified acids which leave the system of the invention through line 36.

Claims: A method of recovering carboxylic acids from a dilute solution thereof, comprising passing the dilute solution through a reverse osmosis membrane separator (26) to produce an aqueous permeate (30) substantially free of acid and an aqueous retentate (28) having an acid concentration more concentrated than the initial concentration of the dilute acid solution, characterised in that the carboxylic acids have from 1 to 7 carbon atoms or are mixtures thereof, the dilute acid solution has an acid concentration below about 10% by weight, and the aqueous retentate (28) has an acid concentration above about 10% by weight, and in that the retentate (28) is treated in an acid separation step (32) by contacting the retentate (28) with a liquid extractant for the acids thereby to produce an acid rich extractate (36) and a substantially acid free raffinate (34), and recovering acid from the acid rich extractate (36). A method as claimed in Claim 1 in which the carboxylic acids have from two to four carbon atoms or are mixtures thereof. A method as claimed in Claim 1 or Claim 2 in which acid is recovered from the acid-rich extractate (36) by distillation. A method as claimed in any preceding claim in which the dilute aqueous solution of carboxylic acids contains one or more impurities consisting of aromatic carboxylic acids, metallic ions, free metal particles, organic or inorganic salts, straight chain carboxylic acids having more than five carbon atoms or sugars and in which these impurities are removed from the dilute aqueous solution prior to its passage through the reverse osmosis separator (26). A method in accordance with Claim 4 in which the impurities are removed from the dilute aqueous carboxylic acid solution by filtration, microfiltration, ultrafiltration, nanofiltration, or a combination of two or more of these processes. A method as claimed in any preceding claim in which the reverse osmosis separator employs a semi-permeable membrane formed from polysulphone, polyether sulphone, cellulose acetate, polyamide, cellulose esters, cellulose triacetate, polyether urea, polyvinyl alcohol, polypiperiazamide, polyfuren, cellulose acetate butyrate, aromatic polyamide, polyethyleneimine, or thin film composites of any of the above.

Inventors: GENTRY JOSEPH CHARLES, GUALY RONALD G, GENTRY, JOSEPH CHARLES, GUALY, RONALD G.
Applicants: GLITSCH, GLITSCH, INC.
Application year: 1994
Main classification: C07C51
IPC codes: C07C53, C07C51, B01D61, B01D11